1. Field of the Invention
The invention relates to an improved leader construction for a photographic film assemblage.
2. Description of the Prior Art
The present invention relates to a photographic film assemblage, preferably of the 35 mm type, and more particularly to such an assemblage which includes an improved leader construction. Generally, film assemblages of this type include a roll of material which is mounted on a spool. The roll of material includes a length of photographic film having a trailing end secured to the spool and a leading end which functions as a leader and is adapted to be attached to a film take-up reel, as described in U.S. Pat. No. 3,690,451. Alternatively, the leading end of the film may be attached to the trailing end of an opaque leader, as described in U.S. Pat. No. 4,295,713. In either case, generally the longitudinally extending opposite sides of the leader and the film are provided with a series of apertures which are adapted to coact with the cogs of a sprocket wheel in a camera to insure that a predetermined length of film and/or leader is withdrawn from the spool during each actuation of the camera's film advance lever.
In film assemblages of the type wherein the leader is opaque and defines at least the ultimate (outermost) convolution and the penultimate (next outermost) convolution of the roll, the aforementioned apertures create light leak problems insofar as it is possible for ambient light to pass through these apertures when they are partially or fully aligned with superposed apertures and fog the underlying film. This problem is especially critical where the spool containing the roll of film and leader is not mounted within a lighttight cassette or housing.
Still another problem with film assemblages of the type described is a phenomenon called clockspringing of the roll of material, i.e., the inherent tendency for the roll to expand as it tries to uncoil. As is well known in the art, such expansion may cause the ultimate or outermost convolution of the roll to move into intimate contact with adjacent surfaces of its housing thereby increasing the forces needed for a film advancing mechanism of a camera to withdraw the film and/or leader from its housing.
Several means have been employed to obviate the disadvantages of clockspringing. For example, the aforementioned U.S. Pat. No. 3,690,451 shows a film leader which has been transversely corrugated such that it makes firm engagement with the surrounding surfaces of its housing. As the film ages, the inherent plasticity causes the effects of clockspringing to diminish as its corrugations relax. U.S. Pat. No. 3,613,876 employs a method wherein a film spool is spot welded to its supporting structure. The welds have sufficient strength to prevent relative rotation therebetween during shipment of the film assemblage, but will break as the film is withdrawn from the spool. Still another method is described in U.S. Pat. No. 3,433,637. In this patent, a roll of film has at least one forwardly directed tab in an inner convolution which is adapted to enter an aperture in the next outermost convolution so as to lock the two convolutions together.
The disadvantages of the aforementioned patent are fairly obvious to those skilled in the art. For example, corrugating the film leader as in U.S. Pat. No. 3,690,451 results in (1) a needless waste of photographic film, (2) a reduction in the amount of usable film which can be housed within the film container due to the increased thickness of the corrugated leader, and (3) the added cost involved in corrugating the film leader. As regard U.S. Pat. No. 3,613,876, the added costs related to the spot welding operation and the attendant tolerances of the welds weigh against its possible adoption as a solution to the problem. The method employed in U.S. Pat. No. 3,433,637 not only entails a separate manufacturing step but also needlessly wastes film.